LONDON, Great Britain - Madison Brengle navigated through a second set hiccup to knock out former World No.2 Petra Kvitova, 6-3, 1-6, 6-2.
Kvitova was playing just her third tournament of 2017, missing six months of action after incurring severe injuries to her left hand during a home invasion.
The two-time Wimbledon champion nonetheless came to the All England Club as an improbably heavy favorite to take the title again after her inspiring win at the Aegon Classic in Birmingham.
Still, Brengle has proven to be tough opposition for Kvitova, having won two of their three previous meetings heading into Wednesday's clash.
The American kicked off 2017 with a surprise win over former World No.1 Serena Williams at the ASB Classic in Auckland, and displayed some of her trademark consistency to edge ahead an opening break and ultimately take the first set.
Kvitova had only played one three-setter heading into Wimbledon - recovering from a set down to defeat Ashleigh Barty in the Birmingham final - and appeared poised to live up to her former "P3tra" nickname as she blasted through the second set to level the match.
Brengle regained control from there, surviving several long games that left the Czech star doubled over as she called the trainer down 2-5 in the final set, having her blood pressure checked before the unseeded American served out the match after an hour and 43 minutes on court.
"My body just didn't really do the best, unfortunately," Kvitova said. "But I was really trying. I fought, and I'm not sure what happened, actually. I could not just breathe, and I was feeling a bit sick, as well.
"That sometimes happens. Unfortunately it happened at Wimbledon, which is not nice. But she still played what she could; she hit everything back. It was just tough for me to still have those energy to put it back and play my aggressive game."
Seeded No.11, Kvitova showed flashes of brilliance behind 36 winners, but was undone by 50 unforced errors; Brengle, by contrast, was far cleaner with her stats (12 winners to 16 unforced errors).
"No one knows how hard I had to work to be back, so probably that's feels more disappointing than normally it is.
"But on the other hand, it's been great to be here again. I mean, it was my goal to play this event this year, and that's what I did. So probably in deep myself I feel very, you know, grateful to have this option to play, but I feel sad that I have to leave so early."
Up next for Brengle is Greek youngster Maria Sakkari, who roared back from a set and 1-4 down to defeat Kristyna Pliskova, 6-7(6), 6-4, 6-4.
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